Got any specifics?
However, I am not surprised.
Were you experiencing any difficulties on the line which prompted the move to the HG612? I suspect it's no coincidence we have access to the QLN and the HLOG on the HG... I guess it uses this info to decide how many bits it can reliably load.
I've no idea what ZyXEL modems do but in my experience, they appear to synch for 'best case' conditions, not what the line can reliably do.
You can probably eek some more speed out of it by playing with the SNRM if you're happy it's uber stable.